Commercial ovens and freezers typically rely on the transmission of a heating or cooling medium such as air to the food product by a large fan or blower. The fan or blower is situated proximate to a conveyor which carries the food through the heater or freezer for a time sufficient to heat or freeze the food product.
The food product entering the oven or freezer has a boundary layer composed primarily of stagnant air which insulates the food product from the surrounding atmosphere. In order to effect proper cooking or freezing, the boundary layer must be substantially reduced to expose the food product directly to the heating or cooling medium.
Conventional fans or blowers generate a multidirectional flow of the heating or cooling vapor. Much of the vapor is scattered about the freezer and only a portion of this scattered vapor reaches the food product. At least a significant portion of the blown vapor, therefore, does not directly impinge on the food product in a perpendicular direction. Under these conditions, the vapor which does contact the food product often does not possess sufficient energy to substantially reduce the boundary layer. This results in inefficient heating or freezing or requires excessively long exposures of the food product to the heating or freezing operation.
Efforts have been made to reduce the amount of heating or cooling vapor which is scattered about the freezer. This has been accomplished by employing a device within the oven or freezer which transforms the multidirectional air flow from the blower or fan into a unidirectional flow of air directly toward the food product which has sufficient energy to reduce the boundary layer.
For example, Donald P. Smith, U.S. Pat. Nos. 3,884,213, 4,289,792 and 4,338,911, disclose a cooking apparatus utilizing a series of spaced apart discrete jets of unidirectionally flowing air produced by appropriately spaced tubes.
Donald P. Smith, U.S. Pat. No. 4,479,776, discloses a heating/cooling apparatus having a thermal treatment zone for supplying columnated jets of a gas to the exterior surface of a food product moving relative thereto in combination with at least one equilibration zone for promoting heat transfer into or out of the interior portions of the food product. A number of vertical spaced apart tubes are positioned in the heating/cooling section to direct a unidirectional air flow toward the food product.
Mitchell C. Henke, U.S. Pat. No. 4,626,661, discloses the use of a plurality of nozzles spaced apart over the pathway of the food product for delivering discrete jets of unidirectional heating/cooling air. A plurality of high velocity air jets are also employed in Steven M. Shei, U.S. Pat. No. 4,757,800, in which impingement apertures direct heated air in a unidirectional manner to heat the food product passing on a conveyor.
Another approach to providing unidirectional flow of air in an oven is disclosed in Virgil L. Archer, U.S. Pat. No. 4,873,107. Instead of employing tubes for directing the heated air toward the food product as discussed above, there is provided a spaced array of rectangular slots. The multidirectional air from the fan or blower is caused to enter the slots and thereby attain a more orderly and direct flow toward the food product. A similar arrangement of rectangular slots is disclosed in Clement J. Luebke et al., U.S. Pat. No. 4,972,824.
Each of these heating/cooling devices provides an improvement over the use of fans and blowers alone because they generally produce a unidirectional flow of heating/cooling air having sufficient energy to reduce the boundary layer of the food product. However, such devices obtain these improvements by expending excessive energy to distribute the heating/cooling air to the food product. In addition, with respect to freezers, the tubes or slots used to form the unidirectional flow often become plugged with frost. The buildup of frost tends to degrade the freezing operation over a period of time. Frost reduces the amount of heat transferred from the food product and, therefore, as the time of the freezing operation increases, the efficiency of the transfer of heat from the food product to the atmosphere decreases. In order to remove the frost to keep the air passageway open, it has been necessary to shut down the freezer to melt the accumulated frost, resulting in delays and additional cost of the process.
Accordingly, it would be desirable to employ a device for transmitting a heating or cooling medium such as air to a substrate such as a food product on a conveyor belt in a more energy efficient manner by providing for better distribution of the heating/cooling medium across the width of the conveyor belt. It would also be of benefit to provide better distribution of the heating/cooling medium from the source (e.g. the fan or the blower) to the food product.
Furthermore, with respect to the freezing of food, it would be of significant benefit to prevent the buildup of frost in the freezer without having to terminate the freezing operation.